For the pact of the Big Three, go here.
The Big Three are the three most powerful gods among the Olympians - Zeus, Poseidon and Hades, the three sons of Kronos and Rhea. While the Big Three gods are the strongest, Zeus is stated to be the most powerful amongst his brothers on multiple occasions. After they had defeated their father with the help of their sisters (Hestia, Demeter and Hera), the Elder Cyclopes and the Hekatonkheires, the three brothers divided the universe between themselves and then drew lots to determine their domains: Zeus received the sky, the heavens, and authority over Mount Olympus, Poseidon received the seas, and Hades received the Underworld. Zeus commands the sky, lightning, kingship, honor, light (Roman form), and summon any weather conditions, Poseidon can manipulate water, storms, and earthquakes, Hades has dominion over the dead and undead, darkness, earth-related disasters, and metals and jewels.
The Earth is split between the three and doesn't belong to any one of them, as all three hold various influences over it and the mortals. Although the Big Three are the most powerful of the gods due to being rulers of their own realms, the other gods aren't to be underestimated, as they all possess great powers. However, that being said, The Big Three are, (especially Zeus) the greatest Gods by a good amount of power.
The powers of the Big Three are directly related: Zeus' control over the weather originates from the Earth's seas, which is Poseidon's domain. Poseidon's control over storms and natural disasters are linked to the weather, which is Zeus' domain. Hades retains his power from people dying while Zeus and Poseidon get their power from people living, fishing, and praying to them.
Demigod Children of the Big Three
A half-blood of the eldest gods
Shall reach sixteen against all odds
And see the world in endless sleep,
The hero's soul, cursed blade shall reap
A single choice shall end his days.
Olympus to preserve or raze.
As a result of how powerful the Big Three are, their demigod children tend to be the most powerful demigods in existence when they are alive, able to shape events through their presence. According to Chiron, World War II was essentially a war between the children of Zeus and Poseidon on one side and the children of Hades on the other. In Percy Jackson and the Sword of Hades, Persephone stated putting the three known existing children of the Big Three together created a powerful enough force that she believed that nothing that they would face could withstand their combined power.
All of the Big Three agreed not to have more demigod children since their oath for fear of the Great Prophecy, but Zeus/Jupiter and Poseidon did. Hades/Pluto already had two children before they swore the oath, so technically he never broke it. The worst he did was never sending Nico and Bianca di Angelo to Camp Half-Blood and Hazel Levesque to Camp Jupiter. Poseidon has Percy Jackson and a Cyclops named Tyson, though there was no fear of Tyson fulfilling the first Great Prophecy as it required a demigod. Zeus/Jupiter had two children: Thalia Grace (a Greek demigod) and Jason Grace (a Roman demigod). It is implied in The Last Olympian that the Big Three might have more demigod children in the future as they are now allowed to have children as part of the deal they made with Percy.
|“||You are children of the three major gods. Who could withstand your combined power?||”|
–Persephone to Thalia, Nico, and Percy in, The Sword of Hades
Hazel Levesque is a Roman demigod daughter of Pluto. She died in Alaska in the 1940’s, but was brought back to life in the early twenty-first century by Nico di Angelo using the Doors of Death. After stopping Gaea from awakening, Hazel was made centurion of the fifth cohort. Following the Battle of San Francisco Bay, she becomes the praetor of the Twelfth Legion Fulminata following the retirement of Reyna Ramírez-Arellano.
Nico and Bianca di Angelo
Hades already had Nico and Bianca when the Big Three made their agreement. Zeus gave Hades a week to send them to Camp Half-Blood, but Hades never did as he knew that Zeus would destroy them the minute they were out of Hades' territory. It was a week after the deadline and Hades was still trying to convince their mother, Maria di Angelo, to let him take them to the Underworld so he could protect them, but Maria repeatedly said no. The last time Hades saw his lover, she was going upstairs to get her purse when Zeus then struck the building with a lightning bolt, completely destroying it; Hades sensed the attack and had enough time to put a shield around Nico and Bianca, but Maria was killed. It was then that he told one of the Furies; Alecto, to take Nico and Bianca to the Lotus Hotel and Casino, where they would be hidden until he sent for them.
When they were found, Bianca joined the Hunters of Artemis while Nico went to Camp Half-Blood. However, Bianca died to save Percy and his friends from a defective version of Talos, leaving Nico as the last child of Hades alive. Nico wandered across the world and the Underworld, orphaned and alone, until he became closer with his father and helped Percy in the Second Titan War. After the Second Giant War in which Nico played a vital part, he chose to remain at Camp Half-Blood and began dating Will Solace, subsequently helping Apollo after he becomes mortal.
Thalia and Jason Grace
Thalia was the first demigod to be born after the Oath, but she ended up becoming the tree that guards Camp Half-Blood for almost seven years. She was resurrected in The Sea of Monsters when her tree was poisoned by Luke Castellan. Percy Jackson, Annabeth Chase, Clarisse La Rue, Grover Underwood, and Tyson retrieved the Golden Fleece from the Sea of Monsters (the Bermuda Triangle).
She later joined the Hunters of Artemis as the goddess' lieutenant, meaning that she wouldn't age and thus couldn't be the half-blood in the Great Prophecy.
Thalia had a Roman younger brother named Jason, but he was abandoned by his mother and taken by Lupa to go train to become a soldier at Camp Jupiter. He helped to stop the awakening of Gaea, but he was later killed by Caligula.
Percy was the second child of the Big Three to be born after the oath. He is the son of Poseidon and Sally Jackson. He originally didn't know anything about his godly heritage. According to Nico di Angelo in The Mark of Athena, Percy is the most powerful demigod that he has ever met which includes the other demigods of the Big Three and the rest of the Seven who are all extremely powerful in their own rights.
Percy's presence was revealed at the worst possible time, as Zeus' Master Bolt and Hades' Helm of Darkness had been stolen. The appearance of a son of Poseidon made Hades and Zeus suspicious, as gods cannot go into each other's territory, but their children can. Poseidon was the only one of the Big Three that didn't have something stolen from him, placing most of the suspicion on him. Percy found both of the items and returned them, successfully preventing a war.
Percy had more adventures and finally saved Olympus from the Titans, with help of Luke and Annabeth, fulfilling the Great Prophecy. Since then, Zeus and Hades have stopped hating Percy and now merely dislike him. Although given that Percy is directly responsible for Hades' position on the council with full privileges and rights, it would be fair to say that Hades has an improved opinion about Percy. He went on to play a major role in the defeat of Gaea and the closing of the Doors of Death, surviving falling into Tartarus itself in the process.
- Chiron mentioned that three is a holy number, as it is the number of the Fates, the Big Three, the Virgin Goddesses, and the Furies.
- Interestingly, the Big Three each have different eye colors: Zeus has electric blue eyes, Poseidon has sea-green eyes and Hades has black eyes. Their hair, in contrast, is the same as the three of them all have black hair.
- All Greek children of the Big Three have black hair, while neither of the Roman children do.
- During World War II, the Allies which representing the British Commonwealth, the Soviet Union of Socialist Republics, and the United States of America were known as the "Big Three".
- The "Big Three" is used as a collective name for the three major allies of World War I; David Lloyd George, George Clemenceau, and Woodrow Wilson. It is currently unknown whether they are demigods.
- Poseidon is the only one of the Big Three to end up with only one demigod child after making the pact.
- Neptune is the only one of the Big Three from either pantheon not to be known to have any current demigod children.
- Neptune is the only member of the Big Three to not appear in any of the books, only being mentioned. Instead, all of the deity's appearances are as Poseidon while both Jupiter and Pluto have appeared.